Wireless telephones, such as mobile/cellular telephones, cordless telephones, and other consumer audio devices, such as mp3 players, are in widespread use. Performance of such devices with respect to intelligibility can be improved by providing noise canceling using a microphone to measure ambient acoustic events and then using signal processing to insert an anti-noise signal into the output of the device to cancel the ambient acoustic events.
Because the acoustic environment around personal audio devices, such as wireless telephones, can change dramatically, depending on the sources of noise that are present and the position of the device itself, it is desirable to adapt the noise canceling to take into account such environmental changes. However, adaptive noise canceling circuits can be complex, consume additional power, and can generate undesirable results under certain circumstances. For example, as depicted in FIG. 1, some noise canceling circuits employ hybrid adaptive noise cancellation, including both: (i) an adaptive feedforward system 102 for generating a feedforward anti-noise signal component from a reference microphone signal ref provided by a reference microphone R and indicative of ambient audio sounds; and (ii) an adaptive feedback system 104 including an adaptive filter 110 and a coefficient control block 112 for generating coefficients for adaptive filter 110, wherein adaptive feedback system 104 generates a feedback anti-noise signal component from a synthesized reference feedback signal synref, the synthesized reference feedback signal based on a difference between an error microphone signal err and an anti-noise signal, wherein the anti-noise signal is equal to the sum of the feedforward anti-noise signal component and the feedback anti-noise signal component, and wherein error microphone signal err is provided by an error microphone E and is indicative of an acoustic output of a transducer 106 (e.g., loudspeaker) and the ambient audio sounds at transducer 106. Before being subtracted from error microphone signal err to generate synthesized reference feedback signal synref, the anti-noise signal is filtered by a secondary path estimate filter 108 for modeling an electro-acoustic path of a source audio signal through transducer 106.
In such approach, synthesized reference feedback signal synref synthesizes the ambient noise seen by error microphone E and is thus independent of the effect of adaptive feedforward system 102. The consequence is that adaptive feedback system 104 is unable to determine the frequency regions that feedforward system 102 has cancelled and adapts to reduce noise in the same regions, causing performance of the adaptive noise cancellation system to suffer.